The present invention relates to an image forming apparatus selectively operable in either a first mode for obtaining a photocopy of an original or a second mode for recording information signals outputted from computers, word processors, facsimile transmitters or the like.
An example of such image forming systems is disclosed in U.S. Pat. No. 4,042,962.
The applicant has found some problems peculiar to such type of image forming systems. That is, when an original is to be copied, the reproducibility of halftone should be superior for ensuring good tone reproduction in the resultant copy image. This is very important, for example, when an original having a halftone such as a photograph is to be copied. This importance can be understood from FIG. 1.
FIG. 1 shows curves representing the relationship between the density D.sub.o of an original and the density D.sub.c of a reproduced (copied) image. These curves will be called D.sub.o -D.sub.c curves. Symbols D.sub.W, D.sub.H1, D.sub.H2 and D.sub.B on D.sub.o axis represent densities in the white area, first and second halftone areas and black area on the original, respectively. In FIG. 1, the curve A shows that the tone reproducibility is satisfactory.
If the D.sub.o -D.sub.c curve follows the curve A, the halftone can faithfully be reproduced in the image. However, if the D.sub.o -D.sub.c curve follows the curve B, the halftone area D.sub.H2 is reproduced into a substantially black area, so that an original including a halftone such as a photograph will be reproduced into very a hard-tone image.
On the other hand, where electrical information signals outputted from a computer, word processor, facsimile transmitter or the like are to be converted into images, the required characteristics of development are very different from the above characteristics of development with respect to reproduction. Most of the images to be outputted in such mode comprise letters, figures, symbols or the like which are to be recorded in the form of binary signals corresponding to white and black colors. It also has been attempted to record the white background, halftone area and black background by the use of trinary signals. In order to obtain a high-quality image having a halftone by the use of such digital signals, there can be used Dither method, Density Pattern method and others which are based on an integration effect in the human's eyes which feel the density of the image from a proportion of black-colored picture elements occupying a very small area.
Thus, the characteristics of development required in the mode in which an image is to be obtained on the basis of information signals do not need to be such as denoted by curve A shown in FIG. 1. However, images having no fog are more severely required than in the copy mode. It can be understood from FIG. 1 that the curve B is preferable rather than the curve A, since the former has a higher latitude with respect to fog.
In the image forming apparatus having a function for copying originals and another function for recordingly converting information signals into images, the use of the same developing device in both of these modes would provide a very hard-tone image where the characteristics of development is set to be optimum for the information recording mode and if an original having a halftone is to be copied. On the contrary, where the characteristics of development is set to be optimum for copying originals having halftone, the latitude would be reduced with respect to fog in the case of information recording.